Glass fibre compositions suitable as reinforcement for composite materials have been described for a long time. In 1966, FR1435073 disclosed a composition for a so-called R-glass fibre comprising 50 to 65 wt % SiO2, 20 to 30 wt % Al2O3, 5 to 20 wt % MgO, and 6 to 16 wt % CaO. These fibres with higher mechanical strength than the more commonly known E-glasses are, however, hindered by rather severe processing conditions, in particular high processing temperatures and more problematic fiberization, which all contribute to their higher cost compared with E-glass.
E-glass is probably the most common type of glass used in continuous fibre reinforced composites. E-glass is defined in ASTM-D-57 8-00 as a glass comprising 52 to 62 wt % SiO2, 12 to 16 wt % Al2O3, 0 to 5 wt % MgO, 16 to 25 wt % CaO, and 0 to 10 wt % B2O3. E-glass is usually used as the reference fibre with which the performance of new glass compositions is compared. Boron free E-glass compositions have been proposed, e.g., in U.S. Pat. No. 5,789,329 with 59 to 62 wt % SiO2, 12 to 15 wt % Al2O3, 1 to 4 wt % MgO, 20 to 24 wt % CaO, and minor amounts of alkaline oxides like Na2O and K2O.
S-glass is a glass generally having higher mechanical strength than E-glasses and comprising about 65 wt % SiO2, 25 wt % Al2O3, and 10 wt % MgO.
A multitude of quaternary glass fibre compositions comprising SiO2, Al2O3, CaO, and MgO as major components, within and around the three main R-, E-, S-glass types revised supra have been proposed in the literature.
GB520246 filed in 1938 discloses a quaternary fibre composition comprising either boron or fluorine in rather high amounts, not admissible nowadays. It also discloses a couple of boron and fluorine free compositions which distinguish themselves from the boron and fluorine containing compositions in a substantially higher amounts of Al2O3. These fibres are not suitable for reinforcement in composite materials as they are designed for use as insulation sheath for electrical cables.
In EP1496026 a glass fibre composition is disclosed comprising substantially less boron than traditional E-glass and yet having a considerably lower viscosity than boron free E-glass formulations, thus reducing batch costs. The glass fibres disclosed in this document comprise, 52-62 wt % SiO2, 10-15 wt % Al2O3, 18-25 wt % CaO, 0.5-4 wt % MgO, and 3.5-5.5 wt % B2O3.
In GB1391384 a glass fibre composition is disclosed comprising no boron and showing acceptable viscosity and liquidus temperature. The glass fibres disclosed in this document comprise, 54-64 wt % SiO2, 9-19 wt % Al2O3, 9-25 wt % CaO, 0-10 wt % MgO, and various oxides in amounts less than 6 wt %.
WO2007/055964 and WO2007/055968 propose glass compositions yielding fibres with higher mechanical properties and enhanced resistance to temperature and acids than E-glasses with good forming capability. They are characterized by a content of 60.5-70.5 wt % SiO2, 10-24.5 wt % Al2O3, and 6-20 wt % of RO (=MgO+CaO+SrO). The glass fibres cited in the examples comprise 4.8-14 wt % CaO, and 5.0-11.3 wt % MgO.
EP1641717 discloses glass fibres with mechanical properties comparable with the ones of R-glass, as disclosed in FR1435073, and yielding enhanced processing properties, in particular, melting and fiberization conditions. The glass fibres proposed in EP1641717 comprise 50-65 wt % SiO2, 12-20 wt % Al2O3, 12-17 wt % CaO, and 6-12 wt % MgO, with a combined amount of (MgO+Al2O3) preferably larger than 24 wt %.
WO2009/138661 teaches that a glass fibre having a high modulus and a liquidus temperature of 1250° C. or less can be obtained with a formulation comprising 50-65 wt % SiO2, 12-23 wt % Al2O3, 1-10 wt % CaO, and 6-12 wt % MgO. The sum of SiO2 and Al2O3 should be greater than 79 wt %.
WO9840321 describes a glass fibre suitable for heat and sound insulation in the building industry comprising 50-60 wt % SiO2, 1-6 wt % Al2O3, 16-22 wt % MgO, and 12-18 wt % CaO. The use of this glass as reinforcement for composite materials is not disclosed.
CA1045641 and U.S. Pat. No. 3,892,581 disclose glass fibres for composites reinforcement comprising 53-57.3 wt % SiO2, 16.3-18.5 wt % Al2O3, 8.5-12.7 wt % CaO, and 6.6-10.5 wt % MgO. Similarly, WO2006/064164 describes glass fibres for reinforcement of composite materials with a higher content of Al2O3 and CaO than the preceding compositions, and comprising 50.0-65.0 wt % SiO2, 12.0-20.0 wt % Al2O3, 12.0-17.0 wt % CaO, and 6.0-12.0 wt % MgO.
WO2008/142347 discloses low cost glass fibre formulations providing excellent trade-off between mechanical properties and production conditions. They comprise rather low combined amounts of (MgO+Al2O3), with 62.0-72.0 wt % SiO2, 4.0-11.0 wt % Al2O3, 8.0-22.0 wt % CaO, and 1.0-7.0 wt % MgO.
WO2011095601, WO2011095598, and WO2011095597 disclose glass fibre compositions having high MgO and CaO amounts, for relatively high MgO/Al2O3 ratios, yielding excellent mechanical properties.
WO2011017405 discloses a number of glass fibre compositions having a relatively high content of SiO2. The few examples containing not more than 59.5 wt. % SiO2 generally yield unacceptably low values of ΔT, with several having even negative values of ΔT (cf. Ex. 35, 42, 47, and 48 of WO2011017405, and annexed FIG. 4).
EP2450321 was filed before, but published after the priority date claimed by the present invention and discloses a number of glass fibre compositions which are close to, albeit different from the ones called for in the present invention.
EP2354104 discloses compositions with pure major oxides (SiO2, Al2O3, CaO, MgO) and no minor oxides, which yield acceptable properties but at a higher cost.
In spite of the extensive research done to date to develop glass fibre compositions with enhanced properties, there still is a need to find glass fibres combining high mechanical, physical and chemical properties with, in particular, production cost effectiveness.